The present improvements were developed to provide users of underground liquid storage tanks with inventory and leak detection information. More specifically, they were developed to provide periodic reconciliation of product flow-through in liquid storage tanks, as well as leak detection during quiescent periods.
The use of sound energy has been proposed previously for measuring distances in a liquid by directing sound waves against reflective surfaces at known locations within the liquid and against the liquid surface. The reflection of such sound waves provides echoes which result in output signals that can be processed to measure liquid depth. When the sound waves are directed vertically within a liquid-containing tank, one can measure the liquid level within the tank, and thereby compute the volume of liquid at a selected time. Various systems for analyzing reflected signals to measure liquid level within a tank have been described in numerous U.S. patents, such as U.S. Pat. Nos. 2,787,160, 3,214,974, and 4,748,846.
A set of reference reflectors spaced vertically at known positions relative to a source of ultrasound energy within a liquid having known physical parameters will provide echoes whose differential timing correlates to the average temperature between the reflectors. However, when working over a vertical distance that demands a series of reflectors, the arrangement of conventional radial tabs or reflectors one above the other will cause the acoustic energy from the transducer to become attenuated by a shadowing effect. While it is recognized that the transmitted acoustic energy in the liquid does bend about each reflector due to a fringe effect, and that the reflectors can be focused and/or stepped to maximize the echo amplitude returned to the transducer, conventional configurations of reference reflectors as evidenced in the above-identified U.S. patents have been found to become limiting in many practical applications of this technology.
Monitoring of product flow-through and leak detection processes in liquid storage tanks requires accurate periodic measurement of liquid level and temperature changes that have occurred over the monitored time. Both the velocity of sound in a liquid and its density are affected by temperature changes that might have occurred. Volume comparisons of liquid at different times can be made by converting actual liquid volume to "net volume" (corrected to a reference temperature, such as 60.degree. C.) or by correcting measured volume to the initial temperature conditions.
Changes in the liquid level that have occurred over the monitored period can be determined by periodic measurement of the transit time or propagation time of sonic energy reflected back to a transducer from the liquid surface. Relative temperature changes that have occurred in the liquid from one measurement time to the next can be determined by measurement of changes in the transit time of the sonic energy reflected back to the transducer from a series of reflectors at known heights throughout the liquid. This temperature information can then be used in calculating volume changes to reconcile flow-through over a period of tank usage or for leak detection over a quiescent period.
Many factors have been identified as contributing to possible errors in both liquid level or volume calculation and leak detection for liquid storage tanks. Effects of temperature on liquid density and sound velocity have the most significant impact. The probe disclosed herein provides a means for determining liquid level and volume, liquid density changes with time and relative densities and liquid temperatures at a substantial number of levels within a tank using periodic measurements of ultrasonic propagation velocity.
The speed of sound through a medium is a direct function of its density and modulus of elasticity. Since density varies with temperature, temperature can be indirectly determined by measuring the speed of sound through the liquid. Changes in density can be measured by determining changes in the propagational velocity of sound over a known distance in the liquid.
When liquid is added to a tank (for example, during a delivery), the temperature of the now-combined liquid in the tank will increase or decrease as it seeks thermal equilibrium with the surrounding environment--atmosphere, backfill, native soil and ground water--about the tank. Similarly, newly-introduced liquid will also seek thermal equilibrium with the liquid previously present in the tank. This equalization process generally has a long time constant and tends toward either thermal stratification or homogeneity, depending on the temperature differential of the two liquids and the surrounding environment.
As a result of developing a two dimensional thermal model of liquids within a tank, it has been determined that thermal knowledge of six inch horizontal sections of the product in a tank would provide adequate temperature information to apply as a correction factor when calculating net volume of the liquid in large storage tanks of the type used for petroleum products. The disclosed probe makes such accurate measurements of propagation time practical in large liquid storage tanks.
The present invention provides a novel probe wherein most of the acoustical energy passes through each reference reflector to subsequent reflectors, while assuring the reflection of an identifiable echo to the transducer. In addition, a novel processing system for the reflected output signals has been developed to effectively convert the reflected output signals to the transit time or propagation time of the reflected energy, which is a direct function of liquid density.